Non-silicone conductive paste for the electrical industry, and its use

ABSTRACT

A conductive paste based on an elastomer of the polyoxypolyolefin type or on a member of the polyolefin family is described, provided with an admixture of conductive particles for the production, in the electronics industry, of a sealing element, of a contact element or of a heat-conduction element. This material is particularly suitable for the production of EMI-RFI gaskets, and, compared with materials known hitherto and based on silicone, has improved performance characteristics and environmental properties.

This is a division of U.S. patent application Ser. No. 09/669,083 filedon Sep. 25, 2000, now U.S. Pat. No. 6,518,496.

BACKGROUND OF THE INVENTION

The invention relates to a conductive paste for the electrical industryand also to its use.

Materials of this type are widely used and are needed in numerousapplications. For example, the material has particular importance inconnection with the sealing of electromagnetically shielded housings inelectronic devices which emit electromagnetic radiation or can bedisturbed by electromagnetic radiation penetrating from outside. Thehousings are produced from an electrically conducting material, or froma material coated so as to be electrically conducting, in order toprovide EMI (electromagnetic interference) and, respectively, RFI (radiofrequency interference) shielding and to improve electromagneticcompatibility (EMC). It is known that the region of the joints at whichthe parts of the housing are joined can also be given shielding by usinggaskets made from an electrically conducting flexible material.

An example of a material of this type is known from U.S. Pat. No.4,011,360. This known material is based on an elastomer, typically asilicone rubber material, with an admixture of electrically conductingparticles. This material polymerizes when exposed to atmosphericmoisture at room temperature.

DE 43 19 965 C2 discloses the use of a material of this type for theproduction of the housings described at the outset. The startingmaterial is extruded as a strand of paste directly in the region of thejoint onto one of the parts of the housing, and polymerizes there toform the gasket. This process is also known to the skilled worker as theformed-in-place-gasket process.

Materials of this type with electrically conducting particles aremoreover used for forming contact points or contact areas, known ascontact pads. They thus take on the function of contact elements.

It is also possible to use thermally conductive particles in order toform what are known as thermal pads. These have the task of dissipating,for example, heat from an electronic component and of passing the sameto a cooling element. One of the most important examples of theapplication is a microprocessor unit in which the heat generated by theprocessor is to be passed to a cooling carrier exposed to a fan.

Despite their undisputed useful properties, materials of this type basedon silicone have some disadvantages.

In this connection a particular problem is that the silicone has theproperty of emitting gaseous materials with short molecular chains.These constituents condense on (adjacent) metal parts or cold sectionsof surfaces. The silicone oil deposit insulates the surface (mostlyundesirably). For example, it has been observed that a deposit of thistype considerably impairs, or destroys, the function of contact relays.Even if the contact is closed mechanically, the deposited silicone oilfilm can completely prevent electrical contact. Despite the recognizedgood properties of silicone-containing materials, they have thereforebeen largely eliminated by the telephone industry.

Another problem is that the silicone oil film drastically impairs theadhesion of the surface. This problem is particularly evident in themotor vehicle industry, where surfaces are frequently painted or coated.Even small amounts of a silicone deposit are sufficient to impair paintadhesion.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a materialof the type mentioned at the outset in which the problems described havebeen eliminated. The material provided should be capable of unrestrictedapplication while retaining the good properties of silicone-basedmaterials as mentioned at the outset.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is based on the concept of using, instead of silicone, anelastomer of the polyoxypolyolefin type or a member of the polyolefinfamily, as in the formula:

where R is alkyl, preferably methyl or ethyl, where R′ is alkyl,preferably methyl or ethyl, and where POP is a polyoxyolefin, preferablypolyoxypropylene.

This material has excellent mechanical properties and is particularlysuitable for the formed-in-place-gasket process to make sealing elementsor damping elements for housings of electronic or telecommunicationsdevices. After polymerization, the sealing or damping elements producedin this way are soft, flexible and nontacky, but they adhereparticularly well to the substrate used. They may be applied eithermanually or by way of computer-controlled equipment or robots.

The excellent properties of the material also mean that it can beprocessed by further processes known per se, such as (mold)casting,spraying, dispensing or printing.

Finally, the material has good environmental compatibility, since it isfree from halogens and nontoxic. In addition, no hazardous gases of anytype are produced in the event of a fire.

Since the novel material is a paste, it is firstly easy to process andcan be shaped as desired by the processing methods described above.Secondly, its viscosity is such that, specifically if theformed-in-place-gasket process is used, the dimensional stability of thestrand produced is sufficient for it to retain its profile without theuse of any additional pressures. The polymerization can therefore takeplace under ambient conditions.

The admixtures of conductive particles represent a filling materialwhich permits the desired property to be achieved in the product byvarying the amount, size and makeup of the particles. An importantissue, besides purely mechanical properties such as the flexibility orthe sealing to be achieved after polymerization, is the conductivityachievable. In this connection, different types of conductivity can bedistinguished for the materials under consideration here, correspondingto the purpose for which they are intended to be used.

One of the most important applications relates to the production ofgaskets on housings, or on sections of housings, printed circuit boardsor the like, in order to achieve shielding with respect tohigh-frequency electromagnetic waves. Examples of admixtures used forthis are nickel powder, silver powder or gold powder or appropriatedusts. However, it is also possible to achieve the shielding effectusing powders or dusts made from other materials, such as those based onaluminum, copper, nickel, iron or steel, as long as these are suitablycovered or coated. Materials which can be used for the covering orcoating are again nickel, silver or gold. It is also possible to usenonconducting particles, such as those based on glass, mica or plastic,as long as these are coated or covered as described above and theelectrical conductivity of the particles which is required for anapplication of this type is achieved.

There is also substantial design freedom with regard to the shape of theparticles. Besides regularly or irregularly shaped compact particles,there is particular interest in fibers which allow the strength of thepolymerized and thus cured final product to be increased significantly.It is clear that, depending on the fiber material used, a suitablecoating or covering must again be present.

Another important group of suitable admixtures is that of particlesbased on graphite, in particular nickel-graphite or ferrite, which areextremely effective in absorbing the electromagnetic waves. These aretherefore used in elements which both damp or seal and have anelectromagnetic action.

A final group of admixtures is used to improve thermal conductivity. Thematerials used for this, for example aluminum oxide (Al₂O₃), boron oxideor magnesium, have particularly high thermal conductivity. These, likethe materials described above, are in the form of particles or fillingmaterials when admixed with the base elastomer.

The structure of the conductive paste is usually that of what is knownas a single-component material, which cures under ambient conditions atroom temperature. The curing time may, if desired, be shortened byexposure to heat, in order to optimize the production process for massproduction. The conductive paste may moreover also be thought of in amanner known per se as a two-component material.

In one specific example of an application it is intended that theconductive paste be used for the production of a flexible gasket for anelectromagnetically shielded housing. Using a controlled-path nozzle, apaste of the material is applied directly to a part of a housing in theregion in which the housing has a joint to be sealed. During dischargeof the plastic material, computer-controlled handling equipment is usedto move the nozzle across the part of the housing. The velocity ofrelative movement of nozzle and housing part is determined by theviscosity of the paste, by the amount and velocity of the materialdischarging from the die, by the cross-sectional area of the diepassage, by the desired cross section of the gasket to be produced andby the makeup of the material.

The strand of material dispensed in this way polymerizes under ambientconditions at room temperature. This procedure takes a relatively longtime, but can be accelerated by controlled exposure to heat.

The starting material used is polyoxypolyolefin with an admixture of apowder made from electrically conducting particles. The material is asingle-component material and cures under ambient conditions at normalroom temperature.

Another important application for these materials is in the productionof contact pads, which may per se have any desired shape, for examplemay be an area or a dot.

A comparable method is used to produce a heat-conducting contactelement, and here the paste comprises an admixture of particles withhigh thermal conductivity. The resultant element has a high level ofthermal conductivity and prevents build-up of heat on the surface used.

1. A contact pad formed of an elastomer and an admixture of conductiveparticles, wherein the elastomer has the general formula

where R and R′ are alkyls and POP is a polyoxyolefin, wherein theadmixture is composed of electrically-conductive particles, and whereinthe electrically-conductive particles comprise a nickel powder, silverpowder, or gold powder.
 2. A contact pad formed of an elastomer and anadmixture of conductive particles, wherein the elastomer has the generalformula

where R and R′ are alkyls and POP is a polyoxyolefin, wherein theadmixture is an aluminum powder, copper powder, nickel powder, ironpowder, steel powder, graphite powder, or nonconductive particles, andwherein the particles are coated or covered with nickel, silver, or goldand are electrically conductive.
 3. The contact pad of claim 2, whereinthe nonconductive particles are fibres made from glass, mica, orplastic.